The effect of fiber type,yarn structure and fabric structure on the frictional characteristics of sock fabrics

Abstract

The objective of this study was to measure the combined effects of fiber type (fine wool, mid-micron wool, acrylic), yarn type (high twist, low twist, single) and fabric structure (single jersey, half-terry, terry) on friction between sock fabrics and a synthetic skin using the horizontal platform method. The effect of weight of a hypothetical wearer and moisture content of a sock fabric were also investigated. Differences among fabrics were analyzed using frictional force traces. Data compared included the static and dynamic friction and coefficients of friction, as well as three new descriptive parameters.

All variables investigated affected the frictional characteristics between a sock fabric and a synthetic skin. Single jersey fabrics had the lowest coefficient of static and dynamic friction. Friction between fabric and a synthetic skin was affected most by the applied weight, with the simulated adult weight resulting in a greater frictional force, and higher coefficients of static and dynamic friction. The most important effect of fiber was on the static frictional force and coefficient of static friction of damp fabrics, with fabrics composed of fine wool exhibiting lowest friction, and acrylic fabrics the highest.

Keywords

sock fabrics fiber type fabric structure yarn type blisters friction

Blister formation on the foot is understood to be influenced by a number of factors including the frictional characteristics of both socks and skin, the pressure between the sock and skin, and skin moisture.^1–4 Factors suggested as altering the frictional characteristics of fabrics are the fiber type,^4–6 applied weight,⁷ yarn density (of woven fabrics),⁸ orientation of the fabric structure (e.g. direction of loops),^9,10 and the presence of moisture in the fabric.^7,11 Approaches used to measure frictional parameters have varied, sometimes using laboratory tests, but more often the effects have been measured indirectly using human trials/wear tests. Though these approaches have contributed to understanding of the frictional characteristics of fabrics, the relative roles of the fabric structure, yarn structure and fiber type of socks are not yet clear.

Determining whether there are differences among commercially manufactured socks of differing fiber type has been a primary focus. For example, wearing acrylic socks reportedly results in fewer and smaller blisters than wearing cotton socks,⁴ although fabric details (e.g. structure, mass, thickness) were not reported. However, Herring and Richie⁴ later repeated their study¹² and found no difference in the incidence of friction blisters between acrylic and cotton socks worn by runners, suggesting that the different cushion soles used in their earlier study,⁴ rather than a “generic sole” used in the later one,¹² may have had a more important effect than the type of fiber alone. The use of commercially available socks/sock fabrics is common (e.g. studies by Herring and Richie⁴ and Euler⁵), which has potential to confound results due to fabric production not being controlled for all variables, thus the effect of individual variables may be difficult to ascertain.

Differing experimental procedures known to affect frictional parameters makes comparisons among published works difficult. A Standard method to measure friction between a fabric and another surface does not exist. Differences in coefficients of friction (µ) between sock fabrics and synthetic skin have been measured using a Textile Friction Analyzer (TFA), results of which have correlated well to preferences as indicated by participants in a human wear trial.⁶ However, this instrument does not appear to be widely available. A more common approach is the horizontal platform method, which involves pulling a weighted sled across a fabric adhered to a platform attached to the lower jaw of a tensile tester.^8,13–16 Despite its frequent use, differences in the experimental set-up have been reported: weight, cross-head speed, contact area, and relative humidity of the environment, all of which affect the reported friction parameters including static and dynamic frictional force, and the coefficients of static and dynamic friction.^13,15

Several variables presumed to affect friction between fabric and another material have also been explored. Of particular interest for sock fabrics in contact with the skin are the pressure exerted by the normal force (i.e. the weight of the wearer) and the effects of moisture content. The coefficients of static and dynamic friction are predicted to decrease as pressure from the normal force increases, a phenomenon that has been observed between fabric surfaces.^8,13,17 Carr et al. examined friction between two fabric surfaces under a range of pressures in the context of separation of fabrics in automated assembly devices, a different objective to that of the present work.¹³ The presence of moisture in fabrics typically results in an increase in frictional force and coefficient of friction and also an increase in regain¹⁸ possibly due to swelling of fibers affecting the fabric surface geometry by increasing the contact points between the fabric and contact surface. However, just one publication has been identified which examined the frictional properties of socks to which an unspecified volume of deionized water had been added to simulate sweating by a foot.⁶ All socks investigated by Bertaux et al. exhibited an increase in µ with the addition of water.⁶

The frictional parameters most commonly reported are the coefficients of static and dynamic friction. Sometimes the static friction and dynamic frictional forces are also reported. However it is acknowledged in the literature that the coefficient of friction for visco-elastic materials, such as textiles, is not a material constant, but instead exhibits a complex relationship with applied pressure, contact area, moisture content, time of contact and method of testing.^17,19 Elucidating the relative contributions of fibers, yarns and fabric structure to the frictional behavior of a fabric in contact with the skin may therefore be enhanced by analyzing the friction force variation trace for parameters in addition to the frictional forces and coefficients of friction. For example, the amplitude of frictional resistance and the number of peaks of the dynamic portion of the friction force variation trace have been reported.^8,20 It may be that additional descriptors of the friction trace could be used to compare fabric behavior in relation to shearing forces, elastic properties, and fabric distortion, thus providing more understanding of how sock fabrics potentially affect blister formation.

The relative roles of fabric structure, yarn and fiber type on characteristics of friction between sock fabrics and skin are not yet known; no study in which all these variables have been controlled has been identified. Therefore the aim of the present study was to determine the relative contributions of fiber, yarn, and fabric structure to frictional characteristics between a synthetic skin and sock fabrics for which the manufacturing process was closely controlled. The effect of weight (of a hypothetical wearer) and moisture content of a sock fabric were also investigated.

Experimental details

Materials and test conditions

A 3 × 3 × 3 factorial experimental design was used to examine the effects of fiber, yarn, and fabric structure (Table 1). 27 fabrics were manufactured (Sangiacomo 4100 HT 12-gauge machine, single cylinder, Christchurch, New Zealand) with production variables including knitting machinery and tension matched as closely as possible across the different yarn and fiber types (Table 2). Typical commercial wet finishing processes were not applied to the fabrics.

Table 1.

Fabric variables

Variable	Description
fiber	fine wool, 19 µm
	mid-micron wool, 26 µm
	acrylic, 19 µm^a
yarn	two-fold conventional, R74/2 tex, singles twist 2700, two-fold at ∼55%
	two-fold low twist, R74/2 tex, singles twist 2100, two-fold at ∼55%
	single conventional, R74/1 tex, singles twist 2700
fabric structure	terry
	half-terry^b
	single jersey

half-terry - loops on every second course only

acrylic density was 1.16 g/cm³, 3 denier fibre has a diameter of approximately 19.1 µm

Acrylic density was 1.16 g/cm³, 3 denier fibre has a diameter of approximately 19.1 µm

Half-terry—loops on every second course only

Table 2.

Sock fabric descriptive properties

Fiber	Yarn twist	Fabric structure	Stitches per 10 mm (course direction)		Mass/unit area (g/m²)		Thickness (mm)
			Mean	s.d.	Mean	s.d.	Mean	s.d.
acrylic	high	single jersey	7.90	0.14	344.80	4.32	1.64	0.05
	low		7.75	0.25	357.40	9.53	1.69	0.04
	single		7.50	0.00	340.60	11.76	1.53	0.03
	high	half-terry	6.70	0.67	449.20	10.71	3.95	0.03
	low		6.30	0.57	446.50	16.38	4.02	0.03
	single		6.70	0.45	468.80	13.03	3.91	0.05
	high	terry	6.60	0.55	515.00	5.66	4.37	0.09
	low		7.00	0.61	522.80	13.77	4.62	0.07
	single		6.50	0.71	551.80	6.76	4.28	0.03

fine-wool	high	single jersey	8.65	0.22	422.40	5.50	1.61	0.01
	low		8.65	0.22	437.60	3.21	1.66	0.02
	single		8.50	0.00	414.60	8.32	1.60	0.03
	high	half-terry	7.10	0.74	572.20	4.21	3.60	0.07
	low		6.60	1.52	563.80	16.89	3.66	0.04
	single		8.05	0.87	568.40	8.96	3.59	0.07
	high	terry	8.45	0.76	653.60	13.58	4.25	0.04
	low		8.50	0.50	687.80	19.01	4.40	0.06
	single		8.50	0.00	678.20	20.13	4.30	0.07

mid-wool	high	single jersey	8.50	0.00	402.20	4.66	1.67	0.04
	low		8.40	0.22	385.40	5.32	1.66	0.04
	single		8.35	0.22	399.20	4.66	1.65	0.02
	high	half-terry	6.90	0.55	532.60	9.71	3.73	0.04
	low		6.70	0.76	498.60	5.32	3.68	0.05
	single		7.00	0.71	534.80	6.14	3.78	0.05
	high	terry	7.60	0.42	633.00	8.46	4.53	0.04
	low		7.60	0.74	607.20	9.55	4.44	0.06
	single		7.40	0.55	627.80	7.73	4.46	0.04

Fabrics were pre-treated to ensure stability in a Wascator washing machine (FOM71MP-Lab, supplied by James Heal and Co Ltd., Halifax, UK) using six cycles of program 8A and detergent A, as specified in BS EN ISO 6330:2001,²¹ and allowed to dry flat after the sixth cycle.²² Fabrics were conditioned for 24 h prior to testing in the Standard environment for conditioning and testing in accordance with EN ISO 139:2005 (i.e. 20.0 ± 2℃, 65.0 ± 4.0% R.H.).²³ Conditions recorded during testing measured using a temperature and R.H. data logger (tiny tag) were, 17.7℃ + 5.4, −3.0 and 73.5% ± 0.80 R.H.

Fabric frictional characteristics were measured using a modified version of the horizontal platform method.^8,13–16,20 A small perspex sled (153.52 g) with a contact area of 60 × 50 mm and curved front edge was loaded with either 2.25 kg to create pressure equivalent to that applied to a sock when worn by a 70 kg adult (6.73 kPa, foot contact area of 0.102 m²) or 1.25 kg to approximate a youth weight (38.6 kg, 3.93 kPa, foot contact area of 0.096 m²), based on 4.05 kPa which has previously been suggested as the pressure exerted by an adult weighing 70 kg with a foot area of 0.017 m² (Figure 1).²⁴ A 60 × 50 mm piece of Lorica Soft® (simulated leather: 49% polyurethane, 49% polyamide microfiber, 2% other; Ehrlich LederHandels GmbH, Biberach, Germany) was cut lengthwise along the cross direction, and was attached (with the cross direction parallel to the long axis of the sled) using double-sided tape to the lower surface of the sled. Lorica Soft® has been reported to correspond best with human skin under dry conditions.²⁵ The Lorica Soft® was replaced for each test. The sock fabric specimen (100 × 300 mm, n = 5) was attached to the platform (550 × 200 mm) technical rear uppermost. An approximate 15% pre-extension was applied to each specimen by clamping one end and attaching a weighted (100 g) metal clip on the free end. The intent of the extension was to subject all specimens to identical set-up conditions. Once extended, a single sheet of double-sided adhesive (100 × 300 mm) was applied to the technical face of the specimen. The weighted clamp was removed from the lower end of the specimen, the adhesive backing removed, and the specimen placed adhesive side-down on the center of the platform (as close as possible to the pulley). The platform was attached horizontally to the lower jaw of an Instron tensile tester (Model 4464, BioLab Limited, Auckland, New Zealand) fitted with a 100 N load cell ± 0.5% reading accuracy, and a pre-load of approximately 1 N applied. The sled was then drawn across the fabric specimen at 250 mm/min using a twisted Kevlar® cord (Tex 400: 4 ply Kevlar® 1000 Denier Type 29 1100 DTEX Merge 1F314, twist singles 6 (S direction), 4 plies twist 3 (Z direction); Texspec New Zealand Limited, Auckland, New Zealand) attached to the upper jaw. Data were recorded as a friction force variation trace (force × time) using an AD Instrument Powerlab (ADInstruments, Dunedin, New Zealand) attached to a PC running LabChart7 software.

Figure 1.

Horizontal platform method set-up.

The test sequence was repeated using damp fabric specimens, evaluated using the “adult” weight (2.25 kg) only. Prior to being adhered to the platform, specimens were made damp by being placed in a Wascator on a wetting cycle (Wascator wetting cycle: 2 kg load, 20℃, rinse gentle action (180 s), drain no action (60 s), extract slow (30 s), drain gentle action (30 s)) as previously reported by Laing et al.,²⁶ and then put immediately inside re-sealable bags for transfer to the conditioned atmosphere (see Table 3 for details). No effort was made to standardize the volume of water held in the specimens, as the treatment was intended to reflect actual use where the fabrics would absorb what they would (e.g. rain, immersion, sweating foot). Specimens remained in the bag until being placed on the platform for testing, no longer than 2 h.

Table 3.

Water absorbed by fabric, expressed as a percentage of dry fabric weight

Fiber	Yarn	Structure	Percentage	s.d.
acrylic	single	jersey	42.40	3.06
acrylic	high	jersey	43.06	3.55
acrylic	single	half-terry	43.62	4.27
acrylic	low	jersey	47.16	5.94
acrylic	low	half-terry	47.62	3.66
acrylic	high	half-terry	48.69	1.95
acrylic	low	terry	48.99	3.07
acrylic	single	terry	49.72	4.70
acrylic	high	terry	51.28	2.19
mid-micron wool	single	terry	67.81	4.54
mid-micron wool	high	half-terry	68.00	3.54
mid-micron wool	low	terry	68.59	5.81
mid-micron wool	low	half-terry	69.68	10.25
mid-micron wool	low	jersey	71.42	3.73
mid-micron wool	single	half-terry	72.22	4.52
mid-micron wool	high	jersey	72.28	12.72
fine wool	low	jersey	73.11	1.67
fine wool	single	jersey	74.71	2.33
fine wool	high	jersey	75.34	6.38
mid-micron wool	single	jersey	77.38	8.05
mid-micron wool	high	terry	77.80	8.78
fine wool	high	half-terry	78.40	3.32
fine wool	high	terry	80.41	4.38
fine wool	low	half-terry	83.55	11.64
fine wool	single	half-terry	85.65	0.70
fine wool	single	terry	88.18	7.86
fine wool	low	terry	88.34	4.16

Data extraction and statistical analysis

Coefficients of static and dynamic friction were calculated from the friction force variation trace¹⁹ using:

\frac{F_{f}}{F_{N}} = μ

(1)

where: µ is the coefficient of friction; F_f is the force of friction in N (recorded from the tensile tester); F_N is the normal force in N (total mass of the sled plus mass added to simulate youth and adult weight respectively in kg × 9.8 m/s² (the acceleration of gravity)).²⁷

The static frictional force was recorded from the highest peak on the friction force variation trace. The mean values of F_f is maximum and F_f is minimum after the highest peak (calculated from the relatively stable segment of the trace from 5–10 s) were averaged and the resulting value was used as the dynamic frictional force (Figure 2).

Figure 2.

Parameters selected for analysis: (a) static frictional force; (b) dynamic frictional force; (c) tangent to the slope; (d) fabric deformation prior to onset of sliding; (e) shape of the force x distance plot.

Additional parameters were selected from the trace for analysis, to explore differences among fabrics that may not be evident from the coefficients of static and dynamic friction. These three additional parameters included: the tangent to the slope of the trace from the start to the static frictional force, the amount of fabric deformation (time was converted to distance) prior to the onset of sliding in the form of loops for which the angle to the fabric’s surface have been decreased or loops which have been compressed by the sled’s movement (considered in this case to be some unknown combination of compression of fabric thickness, distortion of yarn loops and/or stitches, compression of individual fibers and shear deformation of junction points between the fabric and the Lorica Soft® prior to the onset of sled sliding), and the overall shape of the trace (Figure 2).

All force values were adjusted for the baseline (pre-load) for each replicate. The slope from the start of the test to the static frictional force was calculated using nine points and expressed as N/s. In the context of this work, it is referred to as the tangent to the slope. All data were tested for normality and homogeneity (coefficients of static and dynamic friction data were log transformed). Only the fabric deformation prior to the onset of sliding was log transformed to improve normality prior to statistical analysis.

All parameters were analyzed using univariate ANOVA, and data from the dry and damp fabrics were analyzed separately (i.e. via two separate experiments). The overall shape of the trace from the maximum force was also analyzed using repeated measures ANOVA and the Greenhouse–Geisser correction of Mauchley’s sphericity test.²⁸ The static frictional force was standardized to a single value so all traces started from the same maximum. The difference between the maximum static friction and dynamic friction was calculated and reported as a percent change. Data at quarter-second intervals at every 1 s interval were analyzed along the trace for 10 s.

Results

Static frictional force and coefficient of static friction between dry fabrics and synthetic skin

The static frictional force was affected by each of the variables examined: weight, fabric structure, yarn type and fiber type. The heavier weight resulted in approximately double the frictional force of the light weight, regardless of yarn type or fabric structure. The lowest static frictional force and the lowest coefficient of static friction were exhibited in the single jersey fabrics for both weights (see Table 4 and Figure 3, which includes results and observations for both the adult weight and the youth weight as well as both dry and damp conditions). Fabrics composed of acrylic fibers resulted in a greater static frictional force under the heavier weight than both types of wool fabrics (Figure 3).

Figure 3.

Mean values for each fiber, yarn and fabric structure combination by analysis parameter. (a) Static frictional force; (b) dynamic frictional force; (c) coefficient of static friction; (d) coefficient of dynamic friction; (e) fabric deformation prior to onset of sliding; (f) tangent to the slope.

Table 4.

Principal factors affecting frictional parameters

	Dry			Damp
parameters and variables	df	F	Sig.	df	F	Sig.
static frictional force					^a
fiber	2	21.37	0.001	2	126.74	0.001
yarn	2	39.49	0.001	2	9.36	0.001
fabric structure	2	50.61	0.001	2	26.60	0.001
weight	2	10675.29	0.001	–	–	–
fiber * yarn	4	6.16	0.001	4	1.53	NS
fiber * structure	4	10.95	0.001	4	1.59	NS
fiber * weight	4	6.50	0.001	–	–	–
yarn * structure	4	2.15	NS	4	2.15	NS
yarn * weight	4	0.65	NS	–	–	–
structure * weight	4	0.52	NS	–	–	–
coefficient of static friction					^a
fiber	2	16.02	0.001	2	126.74	0.001
yarn	2	38.48	0.001	2	9.36	0.001
fabric structure	2	52.09	0.001	2	26.60	0.001
weight	2	298.52	0.001	–	–	–
fiber * yarn	4	5.06	0.001	4	1.53	NS
fiber * structure	4	8.68	0.001	4	1.59	NS
fiber * weight	4	2.49	NS	–	–	–
yarn * structure	4	1.52	NS	4	2.15	NS
yarn * weight	4	3.16	0.05	–	–	–
structure * weight	4	6.52	0.001	–	–	–
dynamic frictional force					^b
fiber	2	0.33	NS	2	97.12	0.001
yarn	2	15.04	0.001	2	7.44	0.001
fabric structure	2	235.01	0.001	2	279.22	0.001
weight	2	9378.45	0.001	–	–	–
fiber * yarn	4	4.38	0.001	4	0.24	NS
fiber * structure	4	7.81	0.001	4	5.08	0.001
fiber * weight	4	3.21	0.05	–	–	–
yarn * structure	4	0.45	NS	4	1.31	NS
yarn * weight	4	0.73	NS	–	–	–
structure * weight	4	3.74	0.05	–	–	–
coefficient of dynamic friction					^b
fiber	2	0.01	NS	2	97.12	0.001
yarn	2	14.92	0.001	2	7.44	0.001
fabric structure	2	214.86	0.001	2	279.22	0.001
weight	2	335.34	0.001	–	–	–
fiber * yarn	4	3.32	0.01	4	0.24	NS
fiber * structure	4	5.72	0.001	4	5.08	0.001
fiber * weight	4	2.64	NS	–	–	–
yarn * structure	4	0.39	NS	4	1.31	NS
yarn * weight	4	1.89	NS	–	–	–
structure * weight	4	4.22	0.01	–	–	–
fabric deformation prior to onset of sliding
fiber	2	83.28	0.001	2	65.75	0.001
yarn	2	3.35	0.05	2	7.35	0.001
fabric structure	2	1106.25	0.001	2	206.06	0.001
weight	2	77.92	0.001	–	–	–
fiber * yarn		4.60	0.001	4	2.00	NS
fiber * structure	4	30.12	0.001	4	21.59	0.001
fiber * weight	4	2.08	NS	–	–	–
yarn * structure	4	0.24	NS	4	0.19	NS
yarn * weight	4	0.52	NS	–	–	–
structure * weight	4	15.34	0.001	–	–	–
tangent to the slope
fiber	2	25.46	0.001	2	9.01	0.001
yarn	2	1.45	NS	2	9.00	0.001
fabric structure	2	474.69	0.001	2	150.02	0.001
weight	2	658.87	0.001	–	–	–
fiber * yarn	4	1.65	NS	4	0.70	NS
fiber * structure	4	24.42	0.001	4	15.72	0.001
fiber * weight	4	11.11	0.001	–	–	–
yarn * structure	4	0.32	NS	4	1.84	NS
yarn * weight	4	0.28	NS	–	–	–
structure * weight	4	26.60	0.001	–	–	–
shape of the force x distance plot
fiber	2	63.45	0.001	2	7.31	0.001
yarn	2	12.10	0.001	2	2.40	NS
fabric structure	2	370.62	0.001	2	208.23	0.001
weight	2	31.76	0.001	–	–	–
fiber * yarn	4	1.76	NS	4	1.00	NS
fiber * structure	4	10.22	0.001	4	7.00	0.001
fiber * weight	4	2.76	NS	–	3.16	0.01
yarn * structure	4	5.61	0.001	4	3.16	0.01
yarn * weight	4	0.28	NS	–	–	–
structure * weight	4	0.60	NS	–	–	–

a, b

F values are the same as µ calculated from the static frictional force using a constant scaler

†, ††

F values are the same as µ calculated from the static frictional force using a constant scaler

The effect of fabric structure on the static frictional force of fabrics was greater when a light weight was used than a heavy weight (F₂₁₀₈ = 33.56, p ≤ 0.001, F₂₁₀₈ = 18.82, p ≤ 0.001, respectively), when weights were analyzed separately. Also, fiber type had a lesser effect on static frictional force for the light weight than the heavy weight (F₂₁₀₈ = 3.93, p ≤ 0.05, F₂₁₀₈ = 22.37, p ≤ 0.001). Effects of fiber and yarn appear to have a more noticeable effect with a heavier weight. This is likely due to the fact that as the weight increases, the two surfaces will have more contact points;¹⁷ thus the smaller differences among fabrics (e.g. those due to different surface characteristics of fiber and yarn types, contributing to fabric surface roughness) would then become more important.

Fabric deformation before the onset of sliding

The pile loops appear to have strongly affected the extent of fabric deformation prior to the onset of sliding, with half-terry and terry fabrics deforming more when dry and damp than single jersey fabrics. Fabrics deformed more when the heavy weight was applied. Fiber also had a strong effect, with fine wool fabrics exhibiting most deformation when fabrics were dry, while acrylic fabrics deformed most when damp.

Tangent to the slope

Weight had the greatest effect on the tangent to the slope, with the heavier weight resulting in a steeper tangent. The effect of fabric structure on the amount of fabric deformation prior to the onset of sliding in turn affected the tangent to the slope. Fabrics composed of the half-terry structure were observed to have a lower value tangent followed by terry both when fabrics were dry and damp.

Average dynamic frictional force/coefficient of dynamic friction

The dynamic friction force and coefficient of dynamic friction were affected by the applied weight and by fabric structure. Single jersey fabrics exhibited the lowest dynamic frictional force and the lowest coefficient of dynamic friction whether dry or damp, and this is likely due to absence of pile/loops. Yarn also had an effect, with the single yarn resulting in the lowest friction for dry and damp fabrics. Fiber type had no effect on the dynamic frictional force of dry fabrics, but fine wool resulted in the lowest friction/coefficient of friction when fabrics were damp.

Shape of dynamic friction trace (as a percentage of static friction maximum)

Fabric structure had the largest effect on the shape of the dynamic friction curve both when dry and damp (Figures 4 and 5). Single jersey fabrics had the greatest difference as a percentage of the static friction maximum. However, small differences between the different fiber types were also observed. Fabrics composed of acrylic fibers resulted in the greatest difference as a percentage of the static friction maximum when dry, but, when damp, similar values were observed for both acrylic and fine wool.

Figure 4.

Average dynamic frictional force between dry sock fabrics and synthetic skin as percent change from maximum. (a) Fibre type; (b) fabric structure.

Figure 5.

Average dynamic frictional force between damp sock fabrics and synthetic skin as percent change from maximum. (a) Fibre type; (b) fabric structure.

Discussion

Additional analysis parameters

The additional parameters selected for analysis in this study could relate to the formation of blisters, but this study did not elucidate the underlying mechanisms of friction between a sock fabric and synthetic skin. For example, the amount of fabric deformation prior to the onset of sliding could relate to shearing properties, whereas the tangent to the slope could be a measurement of fabric stiffness, visco-elastic properties and/or adhesion.²⁹

Fabric structure

Fabric structure dominated the frictional characteristics between dry fabrics and synthetic skin. Terry structure has previously been reported as having the lowest dynamic coefficient of friction when sliding along the orientation of the fabric, but single jersey had the lowest dynamic coefficient of friction when sliding in the opposite direction by Baussan et al.¹⁰ In the current work, lower frictional forces were consistently observed for single jersey fabrics (possibly due to the fabric surface being smoother, i.e. lack of pile on the technical rear of the single jersey fabric). However, direct comparison with Baussan et al. is not possible for a number of reasons. In the current work, fabrics were tested in one direction only, and, furthermore, Baussan, et al. did not report static friction.¹⁰

The large amount of fabric deformation prior to the onset of sliding exhibited by the half-terry fabrics when dry (Figure 3(e)) may be due differences in pile density. The half-terry open pile structure (e.g. lower packing density with loops on every second row only compared to terry) likely transmitted force across the fabric surface better than the more densely packed full terry fabric, similar to the effect of floating yarns which reduce the coefficient of static friction of woven fabrics.³⁰ Thorndike and Varley did not postulate why floating yarns decreased the coefficient of friction in woven fabrics; however, presumably it is due to the yarns having greater freedom of movement to disperse force, or fewer contact points.³⁰ It could also be due to a change in fabric thickness as fabric loops are compressed or the angle of the loops is decreased with sled movement, thus increasing the number of contact points.

Weight

Weight, the pressure exerted by a simulated wearer affected all frictional parameters investigated, with the effect of weight differing according to fiber type and fabric structure. As pressure between two fabrics increases, a decrease in the coefficient of static and dynamic friction has previously been observed.^8,13 Results from the current study conflict with these findings, instead as the load increased, so did the coefficient of friction, a finding consistent with the work of Sanders et al. in which friction between skin and wool sock fabrics was examined.³¹ The higher coefficient of friction was observed for both static and dynamic friction (Figure 3(c) and (d)). We consider experimental set-up to be a major factor causing differences between the current study and previous work (e.g. by Jeddi et al.⁸ and Baussan et al.¹³), primarily the use of the simulated leather Lorica Soft® as the testing substrate. Lorica Soft® has previously been reported as having a coefficient of friction closest to human skin,²⁵ and this could possibly explain the similarity between our results and those of Sanders et al.³¹ Although the horizontal platform method was used by both Carr et al.¹³ and Jeddi et al.⁸ Jeddi et al. used self-fabric on the bottom of the sled,⁸ whereas Carr et al. used what was described as matching (referred to as “mating” (sic)) fabric, details of which were not reported.¹³ Thus, it is not surprising there would be a difference in µ when using a self-fabric substrate compared to a skin/simulated skin substrate, as the effect of load or weight on µ can vary greatly depending on the properties of the polymers in contact.³² Furthermore, Jeddi et al. did not specify the load applied to the sled,⁸ and Carr et al. used a greater range of pressures (0–35 KPa).¹³ It is reasonable to assume that the differences in set-up explain the difference in results.

Fiber

Acrylic fabrics consistently exhibited the highest frictional forces and coefficient of friction under all conditions. The effect of fiber type did vary according to fabric structure, with the highest frictional forces observed in acrylic half-terry and terry fabrics for both weights and also when fabrics were damp. Differences among the fiber types were less pronounced under the light weight; however, single jersey fine and mid-micron wool generally exhibited the lowest coefficients of friction both when dry and damp.

The effect of fiber type also varied according to fabric structure when the amount of fabric deformation prior to the onset of sliding was analyzed. The differences in deformation prior to the onset of sliding were much larger when fabrics were damp (Figure 3(e)). Damp fine and mid-micron wool fabrics in particular exhibited much less deformation when compared to dry fine and mid-micron wool and also acrylic both when dry and damp. This difference was perhaps due to the hydrophobic nature of acrylic fibers, which may have absorbed less water during the dampening process than wool fabrics (see Table 2, above; the wool fabrics absorbed much higher percentages of water, likely due to water being held both within the fiber and fabric structure). The difference in behavior between wool and acrylic fibers may also be attributable to wool’s decreased bending modulus when wet.^33,34 Fiber bending modulus, in turn, may affect the compressibility of the fabric, thus contributing to changes in the parameter deformation prior to the onset of sliding.

Yarn

Yarn had a lesser effect on frictional characteristics than the other constructional variables, although while often significant, the differences among the yarns were small. The effects of low and high twist yarns were typically similar. The main difference among yarns was the lower coefficient of static friction observed in single yarns for both dry and damp conditions. Thus, the amount of twist in this work was less important than the configuration of the yarn in terms of it being plied or single.

Differences between dry and damp fabrics

Although the experiment was not designed to directly determine whether or not the presence of water was significant, marked differences were observed between dry and damp fabrics. The presence of water increased the maximum coefficient of static friction for all fabrics. Whether water “softened” the surface of the Lorica® substrate as human skin softens when moist is unknown, although a correlation between frictional measurements of damp sock fabrics and Lorica®. Soft using a Textile Friction Analyzer, and participants in a wear trial has been reported by Bertaux, et al.⁶ A lower µ as measured by the TFA correlated with greatest comfort as perceived by participants.⁶

The effect of fiber type became much more pronounced with the presence of water. For example, while fiber type did not affect dynamic frictional force or coefficient of dynamic friction of dry fabrics, it did affect these parameters when fabrics were damp. The greater importance of fiber type when fabrics were damp was expected, based on known fiber properties. No attempt was made to standardize the amount of water, instead fabrics were all subjected to the same “treatment”, reflecting a situation of wear (e.g. rain or walking through water, the fabric will absorb what it absorbs). Fabrics composed of fibers with different hygroscopicity will absorb differing amounts of water (in this study acrylic 42–51%, mid-wool 68–78%, fine wool 73–88%), hence the expected finding.

The amount of fabric deformation exhibited by the fabrics also changed with the presence of moisture. For terry and half-terry fabrics, deformation decreased by 1.00 and 1.28 mm respectively from a dry state to damp, whereas single jersey fabrics increased slightly (0.14 mm) from dry to damp. The presence of water in the loop/terry fabric structure may have decreased the surface roughness of the fabric. The angle at which the loops were oriented in relation to the surface of the fabric may also have been affected by the presence of water in the fabric. Furthermore, the water may have acted as a lubricant between the synthetic skin and the fabric surface, as it is accepted that liquid films reduce stick-slip mechanisms between materials.³⁵ The directionality of the loops may have changed due to the presence of water, as loop direction in relation to the direction of sled movement can affect the coefficient of friction.¹⁰ The water could have interacted with the fabric in a number of ways, none of which were investigated due to the scope of the study. Nevertheless, in this set of experiments the terry fabrics did not transmit force as well when damp.

Conclusions

The primary function of a sock is to act as a barrier between the skin of the foot and a shoe, thus preventing friction blisters and other injuries (e.g. abrasion, impact and pressure). Although the formation of friction blisters is well understood, much of what has been reported on the effect of sock constructional variables on friction between a sock and another material has been speculative. This work has enhanced understanding of relative roles of fiber type, yarn and fabric structure for two different conditions (dry and damp) on frictional parameters. All variables investigated (fabric structure, yarn, fiber and weight) affected the frictional characteristics between a sock fabric and a synthetic skin. Several parameters (e.g. the tangent to the slope of the trace from the start to the static frictional force, the amount of fabric deformation prior to the onset of sliding, and the overall shape of the force variation trace) that appear not to have previously been reported may also affect blister formation, and depending on the parameter investigated, differing constructional variables may be desired.

The presence of moisture and the weight of a wearer are both known to affect frictional forces and thus the formation of blisters. A lower coefficient of friction (thus allowing sliding to take place between the sock and skin) has been reported to reduce shear forces and the formation of blisters.³⁶ If the assumption that the coefficient of friction is the most important frictional parameter on the formation of blisters is correct, fine wool single jersey fabrics had the lowest µ in the current work. However, if the amount of fabric deformation prior to the onset of sliding affects blister formation, when fabrics were dry the ideal fabric would be fine wool low twist half-terry, and when damp would be acrylic half-terry. Even so, the fabric most suitable to reduce the formation of friction blisters will vary depending on the parameter investigated and conditions of use.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Acknowledgments

The Todhunter/Carpenter/Home Science Alumnae Scholarship fund, the University of Otago Christchurch Earthquake Extension fund, the University of Otago Applied Sciences Department and the University of Otago postgraduate publishing bursary are gratefully acknowledged for the provision of tuition fees and/or living stipend.

References

Sulzberger

Cortese

Fishman

. Studies on blisters produced by friction. J Investigat Dermatol 1966; 47(5): 456–465.

Comaish

Bottoms

. The skin and friction: Deviations from Amonton's laws, and the effects of hydration and lubrication. Brit J Dermatol 1971; 84(1): 37–43.

Zhong

Xing

MMQ

Pan

. Textiles and human skin, microclimate, cutaneous reactions: An overview. Cutan Ocul Toxicol 2006; 25(1): 23–39.

Herring

Richie

. Friction blisters and sock fiber composition: A double-blind study. J Podiat Assoc 1990; 80(2): 63–71.

Euler

. Creating “comfort” socks for the U.S. consumer. Knitting Times May 1985, pp. 47–50.

Bertaux

Derler

Rossi

. Textile, physiological, and sensorial parameters in sock comfort. Text Res J 2010; 80(17): 1803–1810.

Kenins

. Influence of fiber type and moisture on measured fabric-to-skin friction. Text Res J 1994; 64(12): 722–728.

Jeddi

AAA

Shams

Nosraty

. Relations between fabric structure and friction. Part 1: Woven fabrics. J Text Instit 2003; 94(3): 223–234.

Baussan

Bueno

M-A

. Rossi RM and Derler S, Experiments and modelling of skin-knitted fabric friction. Wear 2010; 268(9–10): 1103–1110.

10.

Baussan

Bueno

Rossi

. Analysis of current running sock structures with regard to blister prevention. Text Res J 2013; 83(8): 836–848.

11.

Gwosdow

Stevens

Berglund

. Skin friction and fabric sensations in neutral and warm environments. Text Res J 1986; 56(9): 574–580.

12.

Herring

Richie

. Comparison of cotton and acrylic socks using a generic cushion sole design for runners. J Am Podiat Med Assoc 1993; 83(9): 515–522.

13.

Carr

Posey

Tincher

. Frictional characteristics of apparel fabrics. Text Res J 1988; 58(3): 129–136.

14.

Zurek

Jankoiak

Frydrych

. Surface frictional resistance of fabrics woven from filament yarns. Text Res J 1985; 55(2): 113–121.

15.

Hermann

Ramkumar

Seshaiyer

. Frictional study of woven fabrics: The relationship between the friction and velocity of testing. J Appl Polym Sci 2004; 92(4): 2420–2424.

16.

Ramkumar

Umrani

Shelly

. Study of the effect of sliding velocity on the frictional properties of nonwoven fabric substrates. Wear 2004; 256(3–4): 221–225.

17.

Gupta

El Mogahzy

. Friction in fibrous materials. Part I: Structural model. Text Res J 1991; 61(9): 547–555.

18.

Morton WE, Hearle JWS. Fibre friction. In: Physical Properties of Textile Fibres. 3rd edition. Manchester, UK: The Textile Institute 1993, p.635.

19.

Ramkumar

Rajanala

Parameswaran

. Frictional characterization of polymeric materials: A review. AATCC Review 2005; 5(2): 17–20.

20.

Ajayi

. Fabric smoothness, friction and handle. Text Res J 1992; 62(1): 52–59.

21.

ISO 6330:2000. Textiles—Domestic washing and drying procedures for textile testing. Geneva, ISO.

22.

Gore

Laing

Wilson

. Standardizing a pre-treatment cleaning procedure and effects of application on apparel fabrics. Text Res J 2006; 76(6): 455–464.

23.

ISO 139:2005. Textiles—Standard atmospheres for conditioning and testing. Geneva, ISO.

24.

Dayiary

Najar

Shamsi

. An experimental verification of cut-pile carpet compression behavior. J Text Instit 2010; 101(6): 488–494.

25.

Derler

Schrade

Gerhardt

L-C

. Tribology of human skin and mechanical skin equivalents in contact with textiles. Wear 2007; 263(7–12): 1112–1116.

26.

Laing

Wilson

Gore

. Determining the drying time of apparel fabrics. Text Res J 2007; 77(8): 583–590.

27.

Howell

. The laws of static friction. Text Res J 1953; 23(8): 589–591.

28.

Scheiner

Gurevitch

. Design and Analysis of Ecological Experiments, New York: Oxford University Press, 2001.

29.

Gupta

Ajayi

Kutsenko

. Experimental methods for analyzing friction in textiles. In: Friction in Textile Materials, Cambridge, UK: Woodhead Publishing, 2008, pp. 177–177.

30.

Thorndike

. Varley L, Measurement of the coefficient of friction between samples of the same cloth. J Text Instit Proc 1961; 52(6): P255–P271.

31.

Sanders

Greve

Mitchell

. Material properties of commonly-used interface materials and their static coefficients of friction with skin and socks. J Rehab Res Dev 1998; 35(2): 161–176.

32.

Myshkin

Petrokovets

Kovalev

. Tribology of polymers: Adhesion, friction, wear, and mass-transfer. Tribol Int 2006; 38(11): 910–921.

33.

Morton WE, Hearle JWS. Tensile properties. In: Physical Properties of Textiles Fibres. 3rd edition. Manchester, UK: The Textile Institute, 1993: 406.

34.

Chapman

. The bending stress-strain properties of single fibres and the effect of temperature and relative humidity. J Text Instit 1973; 64(6): 312–327.

35.

Berman

Ducker

Israelachvili

. Origin and charaterization of different stick-slip friction mechanisms. Langmuir 1996; 12(19): 4559–4563.

36.

Dai

X-Q

Zhang

. Effect of sock on biomechanical responses of foot during walking. Clin Biomech 2006; 21(3): 314–321.